1. Field of the Invention
The present invention pertains to quasi-crystalline boehmites containing additives.
2. Description of the Prior Art
Alumina, alpha-monohydrates or boehmites and their dehydrated and or sintered forms are some of the most extensively used aluminum oxide-hydroxides materials. Some of the major commercial applications, for example, ceramics, abrasive materials, fire-retardants, adsorbents, catalysts fillers in composites, and so on, involve one or more forms of these materials. Also, a substantial portion of commercial boehmite aluminas is used in catalytic applications such as refinery catalysts, catalyst for hydroprocessing hydrocarbon feeds, reforming catalysts, pollution control catalysts, cracking catalysts. The term “hydroprocessing” in this context encompasses all processes in which a hydrocarbon feed is reacted with hydrogen at elevated temperature and elevated pressure. These processes include hydrodesulphurisation, hydrodenitrogenation, hydrodemetallisation, hydrodearomatisation, hydro-isomerisation, hydrodewaxing, hydrocracking, and hydrocracking under mild pressure conditions, which is commonly referred to as mild hydrocracking. This type of alumina is also used as a catalyst for specific chemical processes such as ethylene-oxide production and methanol synthesis. Relatively more recent commercial uses of boehmite types of aluminas or modified forms thereof involve the transformation of environmentally unfriendly chemical components such as chlorofluorohydrocarbons (CFCs) and other undesirable pollutants. Boehmite alumina types are further used as catalytic material in the combustion of gas turbines for reducing nitrogen oxide.
The main reason for the successful extensive and diversified use of these materials in such variety of commercial uses is their flexibility, which enables them to be tailor-made into products with a very wide range of physical-chemical and mechanical properties.
Some of the main properties which determine the suitability of commercial applications involving gas-solid phase interactions such as catalysts and adsorbents are pore volume, pore size distribution, pore texture, specific density, surface areas, density and type of active center, basicity and acidity, crushing strength, abrasion properties, thermal and hydrothermal aging (sintering), and long-term stability.
By and large, the desired properties of the alumina product can be obtained by selecting and carefully controlling certain parameters. These usually involve: raw materials, impurities, precipitation or conversion process conditions, aging conditions and subsequent thermal treatments (calcination/steaming), and mechanical treatments.
Nevertheless, in spite of this wide and diversified range of existing know-how, this technology is still under development and presents unlimited scientific and technological challenges to both the manufacturers and the end-users for further development of such alumina-based materials.
The term boehmite is used in the industry to describe alumina hydrates which exhibit XRD patterns close to that of aluminum oxide-hydroxide [AlO(OH)], naturally occurring boehmite or diaspore. Further, the general term boehmite tends to be used to describe a wide range of alumina hydrates which contain different amounts of water of hydration, have different surface areas, pore volumes, and specific densities, and exhibit different thermal characteristics upon thermal treatment. Yet although their XRD patterns exhibit the characteristic boehmite [AlO(OH)] peaks, their widths usually vary and they can also shift location. The sharpness of the XRD peaks and their locations have been used to indicate the degree of crystallinity, crystal size, and amount of imperfections.
Broadly, there are two categories of boehmite aluminas. Category I, in general, contains boehmites which have been synthesized and/or aged at temperatures close to 100° C., most of the time under ambient atmospheric pressure. In the present specification, this type of boehmite is referred to as quasi-crystalline boehmite. The second category of boehmites consists of so-called micro-crystalline boehmites.
In the state of the art, category I boehmites, i.e. quasi-crystalline boehmites, are referred to interchangeably as: pseudo-boehmites, gelatinous boehmites or quasi-crystalline boehmites (QCBs). Usually, these QCB aluminas have very high surface areas, large pores and pore volumes, and lower specific densities than microcrystalline boehmites. They disperse easily in water of acids, have smaller crystal sizes than micro-crystalline boehmites, and contain a larger number of water molecules of hydration. The extent of hydration of the QCB can have a wide range of values, for example from about 1.4 up, and about 2 moles of water per mole of Al0, usually intercalated orderly or otherwise between the octahedral layers.
The DTG (differential thermographimetry) curves of the water release from the QCB materials as a function of temperature show that the major peak appears at much lower temperatures compared to that of the much more crystalline boehmites.
The XRD patterns of QCBs show quite broad peaks, and their half-widths are indicative of the crystal size as well as the degree of crystal perfection.
The broadening of the widths at half-maximum intensities varies substantially and for the QCBs typically can be from about 2°–6° to 2θ. Further, as the amount of water intercalated in the QCB crystals is increased, the main (020) XRD reflection moves to lower 2θ values corresponding to greater d-spacings. Some typical, commercially available QCB's are: Condea Pural®, Catapal® and Versal® products.
The category II boehmites consist of microcrystalline boehmites (MCBs), which are distinguished from the QCBs by their high degree of crystallinity, relatively large crystal sizes, very low surface areas, and high densities. Unlike the QCBs, the MCBs show XRD patterns with higher peak intensities and very narrow half-peak line widths. This is due to the relatively small number of intercalated water molecules, large crystal sizes, higher degree of crystallization of the bulk material, and smaller amount of crystal imperfections present. Typically, the number of intercalated molecules of water can vary from about 1 up to about 1.4 per mole of Al0. The main XRD reflection peaks (020) at half-length of maximum intensity have widths from about 1.5 down to about 0.1 degree 2-theta (2θ). For the purpose of this specification we define quasi-crystalline boehmites as having 020 peak widths at half-length of the maximum intensity of 1.5 or greater than 1.5°. Boehmites having a (020) peak width at half-length of maximum intensity smaller than 2 are considered micro-crystalline boehmites.
A typical commercially available MCB product is Condea's P-200® grade of alumina. Overall, the basic, characteristic differences between the QCB and MCB types of boehmites involve variations in the following: 3-dimensional lattice order, sizes of the crystallites, amount of water intercalated between the octahedral layers, and degree of crystal imperfections.
As for the commercial preparation of these boehmite aluminas, QCBs are most commonly manufactured via processes involving:
Neutralization of aluminum salts by alkalines, acidification of aluminate salts, hydrolysis of aluminum alkoxides, reaction of aluminum metal (amalgamated) with water, and rehydration of amorphous rho-alumina obtained by calcining gibbsite. The MCB types of boehmite aluminas in general are commercially produced by hydrothermal processes using temperatures usually above 150° C. and autogeneous pressures. These processes usually involve hydrolysis of aluminum salts to form gelatinous aluminas, which are subsequently hydrothermally aged in an autoclave at elevated temperatures and pressures. This type of process is described in U.S. Pat. No. 3,357,791. There are several variations on this basic process involving different starting aluminum sources, additions of acids or salts during the aging, and a wide range of process conditions.
MCBs are also prepared using hydrothermal processing of gibbsite. Variations on these processes involve: addition of acids, alkaline metals, and salts during the hydrothermal treatment, as well as the use of boehmite seeds to enhance the conversion of gibbsite to MCB. These types of processes are described in Alcoa's U.S. Pat. No. 5,194,243, in U.S. Pat. No. 4,117,105 and in U.S. Pat. No. 4,797,139.
Nevertheless, whether pseudo-, quasi- or microcrystalline, such boehmite materials are characterized by reflections in their powder X-ray. The ICDD contains entries for boehmite and confirms that there would be reflections corresponding to the (020), (021), and (041) planes. For copper radiation, such reflections would appear at 14, 28, and 38 degrees 2-theta. The various forms of boehmite would be distinguished by the relative intensity and width of the reflections. Various authors have considered the exact position of the reflections in terms of the extent of crystallinity. Nevertheless, lines close to the above positions would be indicative of the presence of one or more types of boehmite phases.
U.S. Pat. No. 5,972,820 (Kharas) discloses the preparation of δ-alumina from a pseudo-boehmite. The only alumina compound disclosed in this document that contains other components, such as promoters, activators, and catalytically active metals (col. 8, line 57 to column 10, line 21), is delta-alumina (δ-alumina). As illustrated in col. 5, line 9, δ-alumina is an intermediate in the transformation of boehmite (AlOOH), via gamma-alumina to alpha-alumina. In this transformation, δ-alumina is an intermediate between gamma- and alpha-alumina. Since δ-alumina is derived from a boehmite precursor it is not in itself a boehmite or (pseudo)boehmite.
Pages 225–227 of the Kirk Othmer Encyclopedia of Chemical Technology (Third Edition, Vol. 2, 1978), shows in FIG. 5 that a gamma, delta, eta and alpha alumina are formed by high-temperature decomposition of boehmite, the decomposition sequence being equal to the sequence presented by Kharas. This is further indication that δ-alumina is a decomposition product of boehmite and therefore a different type of alumina than boehmite.
U.S. Pat. No. 6,027,706 (Pinnavaia) discloses the preparation of a synthetic mesostructured alumina composition from, e.g., pseudo-boehmite (col. 8, lines 18–24 and column 17, scheme 4). It is further mentioned that the mesostructured alumina compositions can be impregnated with several metals. An XRD pattern of such a mesostructured alumina is presented in, e.g., FIGS. 1A, 1B, and 10. These patterns were measured using copper radiation (col. 18, line 34).
As mentioned above, the (020) reflection of boehmite appears at 14 degrees 2-theta when using copper radiation. In the XRD patterns of Pinnavaia, no peak can be identified at 14 degrees 2-theta. Hence, Pinnavaia's mesostructured aluminas do not show the (020) boehmite reflection and also differ from the quasi-crystalline boehmites according to the present invention.
In the prior art, we find QCBs containing metal ions which have been prepared by hydrolysis of alumina isopropoxide with co-precipitation of lanthanides, as described in the paper by J. Medena, J. Catalysis, Vol. 37 (1975), 91–100, and J. Wachowski et al., Materials Chemistry, Vol. 37 (1994), 29–38. The products are pseudo-boehmite type aluminas with the occlusion of one or more lanthanide metal ions. These materials have been used primarily in high-temperature commercial applications where the presence of such lanthanide metal ions in the pseudo-boehmite structure retards the transformation of the gamma-alumina to the alpha-alumina phase. Therefore, a stabilization of the gamma phase is obtained, i.e. a higher surface area is maintained before conversion to the refractory lower surface area alpha-alumina. Specifically, Wachowski et al. used the lanthanide ions (La, Ce, Pr, Nd, Sm) in quantities from 1% to 10% by weight, calcined at temperatures in the range of 500° C. to 1200° C. No information is provided by Wachowski et al. regarding the state and properties of the materials below 500° C., which is the most important area for catalytic applications.
Also, EP-A1-0 597 738 describes the thermal stabilization of alumina by the addition of lanthanum, optionally combined with neodymium. This material is prepared by aging flash-calcined Gibbsite in a slurry with a lanthanum salt at a temperature between 70 and 110° C., followed by a thermal treatment at a temperature between 100 and 1000° C.
These products, like the products produced by Wachowski et al., all are high-temperature refractory (ceramic) materials which because of their bulk structures of extremely high density, very low surface areas, and small pores find very limited application in heterogeneous catalysis, especially for catalysts used in hydrocarbon conversion or modification, for example FCC and hydroprocessing commercial applications.
Further, EP-A-0 130 835 describes a catalyst comprising a catalytically active metal supported on a lanthanum or neodymium-β-Al2O3 carrier. Said carrier is obtained by the precipitation of aluminum nitrate solution with ammonium hydroxide in the presence of a lanthanum, praseodymium or neodymium salt solution. As the precipitated amorphous material is directly washed with water and filtered, the alumina is not allowed to age with time under the usual conditions and a certain pH, concentration, and temperature, so that it crystallizes to a boehmite alumina structure.